The production of alkali metal-containing silicate glass foams has been disclosed in the prior art. One description of such can be found in U.S. Pat. No. 3,498,802. That patent discloses the hydration of alkali metal silicate glass powders in steam atmospheres. The primary teaching of the patent is that water migrates into the glass structure to thereby impart thermoplastic properties thereto and to produce products demonstrating the characteristics of hydraulic cements. The utility of the invention encompassed a wide range of glass compositions, viz, about 6-20 mole percent Na.sub.2 O and/or K.sub.2 O and 80-94 mole percent SiO.sub.2, the sum of those components constituting at least 90% of the total composition. Such compatible metal oxides as Al.sub.2 O.sub.3, B.sub.2 O.sub.3, BaO, MgO, PbO, and ZnO were stated to be satisfactory optional ingredients, whereas CaO and Li.sub.2 O were preferably entirely absent from the compositions.
The process of the invention involved subjecting glass powders to a gaseous atmosphere containing at least 50% by weight steam at a pressure of at least 1 atmosphere and a temperature customarily and preferably between about 100.degree.-200.degree. C. The hydration treatment was conducted for a sufficient length of time to produce at least a surface layer on the glass powders having a water content of up to about 30% by weight.
Whereas the main emphasis of the patent is concerned with the thermoplastic behavior that the hydrated glass can display because of the water incorporated into the structure thereof, the formation of foamed masses is also disclosed via heating the hydrated material at temperatures between about 80.degree.-150.degree. C. Such heating dehydrates the glass with concomitant foaming as the water escapes out of the glass structure.
However, certain composition regions within the description of that patent demonstrated such poor chemical durability and weathering resistance that, unless the surfaces thereof were protected from the ambient environment, articles made therefrom quickly lost their integrity. U.S. Pat. No. 3,811,853 discloses the manufacture of articles which can profitably utilize this character of poor durability and resistance to weathering. That patent describes the hydrating of glass articles, particularly containers, consisting essentially, by weight, of about 10-30% Na.sub.2 O and/or K.sub.2 O and 65-90% SiO.sub.2, the sum of those components constituting at least 80% of the total composition. Thereafter, means are provided for securing a weathering-resistant surface on the articles. A self-degradation reaction begins immediately and spontaneously when the weathering-resistant surface is penetrated or removed, since that act exposes the poorly-durable, hydrated interior portion of the article to the ambient atmosphere.
U.S. Pat. No. 3,912,481 provides a method for producing hydrated alkali metal-containing silicate glass articles that can demonstrate good thermoplastic properties, and which also will manifest relatively good chemical durability and resistance to weathering. The disclosed method contemplates a two-step procedure wherein first a glass article consisting essentially, in mole percent, of about 3-25% Na.sub.2 O and/or K.sub.2 O and 50-95% SiO.sub.2, the sum of those ingredients comprising at least 55% of the total composition, is hydrated in a saturated or nearly saturated steam environment at elevated temperatures and pressures. Subsequently, the article is partially dehydrated in an atmosphere of lower relative humidity at elevated temperatures. The principal thrust of the invention is to control the amount of water removed from the glass during the dehydration process such that adequate water will remain within the glass structure to impart thermoplastic properties thereto, but sufficient water will have been eliminated to result in a product having substantially improved chemical durability and weathering resistance, when compared with that exhibited by the fully hydrated glass. The patent emphasizes that care must be exercised during the dehydration step to avoid foaming of the glass. The compatible metal oxides Al.sub.2 O.sub.3, B.sub.2 O.sub.3, BaO, MgO, PbO, and ZnO are noted as being operable additives to the base alkali metal silicate glass.
The utility of water as a foaming agent to produce foamed masses of alkali metal silicate glasses is well-known to the prior art. Several recent U.S. patents utilizing that phenomenon include U.S. Pat. Nos. 3,663,249, 3,725,095, 3,743,601, and 3,756,839. Nevertheless, such foamed articles, when present as a simple glass composition, have not demonstrated adequate stability to be of practical service. Hence, numerous inventions have been proposed involving the addition of various additives to improve the durability, i.e., to "insolubilize" the foams.
U.S. application Ser. No. 641,025, filed concurrently herewith in the names of P. E. Blaszyk, W. T. Brydges, III, E. J. Illig, W. H. Tarcza, and C. K. Wu, now abandoned describes the manufacture of foamed glass articles manifesting good mechanical strength, a high degree of closed porosity, low friability, and good chemical durability. In general, the method comprises hydrating particles of glass having compositions within a very narrow range of the Na.sub.2 O-K.sub.2 O-Al.sub.2 O.sub.3 -ZnO-SiO.sub.2 system at elevated temperatures and pressures in saturated or nearly saturated steam environments to such an extent that the individual particles coalesce together into an integral mass. Thereafter, the integral mass is dehydrated in a manner designed to cause foaming of the glass as the water escapes from the structure thereof. The resultant foamed articles display such good chemical durability and weathering resistance that additives to "insolubilize" the articles are not required.